WASHINGTON — The chairman of the House Iran-contra committee said Sunday that if it is proved a ''smoking gun'' memo written by Lt. Col. Oliver North reached President Reagan, lawmakers would call for impeachment proceedings against Reagan.

Rep. Lee Hamilton, D-Ind., was questioned on ABC's This Week with David Brinkley about the memorandum, a National Security Council decision paper for Reagan drafted by North in April 1986.

Fawn Hall, North's secretary, has testified that a copy of the memo was sent to national security adviser John Poindexter and had been revised.

A version of the memo, found by Justice Department investigators days before the scandal broke, included a proposal to divert $12 million from one of the arms sales to Iran to the Nicaraguan contras.

Reagan has denied all knowledge of the diversion and a link between him and the memo has not been shown.

''I think if that memo had reached the hands of the president and he had approved it, that would be the smoking gun,'' Hamilton said. ''I don't have any doubt at all that that kind of evidence would be exceedingly serious for the president.

''I think it is likely, if that occurred -- and I emphasize the 'if' -- you would have a demand for impeachment proceedings.''

Hamilton had delivered a lengthy speech last week on the ''extraordinary'' testimony he has heard during the six weeks so far of the Iran-contra hearings.

He was on the Brinkley show with other committee members Sen. George Mitchell, D-Maine, and Rep. Henry Hyde, R-Ill.; and with retired Maj. Gen. John Singlaub, one of the private citizens running the supply network for the contras.

Mitchell and Hyde refused to compare the Iran-contra affair to the Watergate scandal that ended with calls for impeachment and the resignation of President Nixon but said that Reagan has been ''hurt badly.''

Mitchell said, ''The lack of wisdom of the sale of arms to Iran and the exchange of arms for hostages has been evident to everyone. . . . I don't think the president will ever regain the status he had prior to last November's election, but he still can be an effective force under certain circumstances.''

Hyde put it this way: ''He is indeed a lame duck and I'd hate to have the last memory of the Reagan administration, the two of them, be this Iran-contra controversy because it's nothing anyone can be proud of.''

Singlaub estimated the contras would need more than $100 million in U.S. aid to win their war against Nicaragua's Sandinista government. He said he was ''dismayed'' to learn from testimony in the hearings that $8 million in a Swiss bank account kept by retired Air Force Maj. Gen. Richard Secord never reached the contras.

''There were times, and even right now, when those funds could be used to great advantage to help the Nicaraguan freedom fighters,'' Singlaub said. ''And to think that I was breaking my back to find funds around the world, when there were already that number of dollars in a Swiss bank account, is very irritating.''

Singlaub called the congressional cutoff of U.S. aid to the contras an ''aberration,'' and compared his private fund-raising network for the rebels to those run by the French, Dutch and Spanish during the American Revolution. The retired general said he and his network did not violate any laws. ''When I got involved in helping get weapons, I found out what the laws were and took steps to make sure that I did not violate any laws of this country.''